There is a general need in the art for a superior method for the selective adsorption of gases on the basis of their size difference. Zeolites have been successfully used as molecular sieves for this purpose due to their pore size being similar to the typical size of the gases being separated. Modification of the pore sizes of these zeolites is typically achieved by the exchange of cations. For example, an A zeolite with Na cations has a pore aperture of ˜4 Å. Ion exchanging the Na cations with K or Ca results in a pore size of ˜3 and 5 Å, respectively. However, this method of pore size modification has its limitations in that it is not as effective for separating molecules whose size difference falls within that which an ion exchange can achieve. For example, commercially available 4 Å, also known as NaA, has a pore size of ˜4 Å which is large enough to adsorb both N2 and CH4, which have kinetic diameters of 3.64 and 3.80 Å, respectively. Correspondingly, commercially available 3 Å zeolite, which contains ˜40-60% K balance Na, offers a pore size closer to 3 Å, which is too small to adsorb either N2 or CH4. Therefore, a method is needed which can fine tune the effective pore mouth opening of the zeolite which can subsequently improve the selectivity of one gas over another, which in this example is N2 over CH4.
Another separation where the method of the invention can prove quite useful is CO2 separation from CO gas, such as the removal of CO2 from a syngas containing CO. Since CO2 and CO have kinetic diameters of 3.3 and 3.7 Å respectively, the same situation occurs where 4A zeolite readily adsorbs both gases and 3A zeolite has a pore mouth which is too small to adsorb either component.
Still another separation is CO2 separation from N2 gas having the aforementioned kinetic diameter differences. This invention will have a strong benefit especially in the case of high removal rates of CO2 to low volume fractions (<10%) where N2 or CO can co-adsorb and reduce the available adsorption sites on traditional adsorbents but will be limited in this present invention.
The teachings of the prior art have addressed the use of silica as a means of coating a zeolite surface to modify the existing pore size. However, the teachings have been limited either in the scope of the pore size change and/or in the method to which the pore is reduced. Those skilled in the art will recognize that previous teachings do not address the specific recipe or processing conditions contained in this patent that are used to control the ultimate effective pore size.
U.S. Pat. No. 6,878,657 controls pore aperture size of zeolite A depositing a silica coating on the external surface of the zeolite. Specifically, the sorption of several gases including nitrogen, oxygen, and argon on silica treated zeolite A was studied. Sorption of these gases by zeolite A which was treated by various quantities of tetraethyl orthosilicate (TEOS) showed that sorption of argon, nitrogen, and oxygen decreased with increasing silica coating with the effects greatest for argon and least for oxygen. While this patent teaches the separation of O2 from N2 and argon, it does not recognize any real benefit in separating nitrogen and argon. The present invention amongst other things allows for the separation of nitrogen and methane, despite their very close size difference (3.64 vs. 3.80 Å). The sample preparation is also significantly different in that the U.S. Pat. No. 6,878,657 stresses the need to pre-dry the zeolite before introducing the TEOS in dry toluene. The present invention uses silicone resin emulsion which coats among other materials a zeolite powder which can be dried. Another important distinction of the present versus the prior art is the fact that the silicone resin coating used in the present invention can also act as a binding agent for the composition for agglomeration. In the prior art, the amount of coating of TEOS taught for the effective pore size reduction is sufficient to effectively bind agglomerates and provide sufficient crush strength. Accordingly the amount of TEOS used in the formulation can range up to 1% of the zeolite weight used. If the average crystal size of the 4A zeolite is 2 microns, then using 1% by weight TEOS would equate to an average crystal coating thickness of 120 Å of TEOS before calcination. In the present invention, the amount of silicone resin used in the examples would be enough to coat the 4A crystals with an average of 980 Å, assuming similar 2 micron sized crystals.
WO 2010/109477 A2 discloses the selective separation of carbon dioxide from a gaseous mixture with nitrogen. The adsorbent material is prepared by pre-drying zeolite A powder followed by treatment with tetra alkyl ortho silicate dissolved in dry solvent. The coated zeolite is then calcined to convert the silicate coating to silica. A second embodiment for the zeolite includes cation exchange to potassium which decreases the A pore size to ˜3 Å and allows for the separation of CO2 and N2. The present invention differs in that the treatment method for the pore mouth modification coating of the zeolite includes silicone resin coating of the undried zeolite. The present invention also does not find the necessity of an ion exchange for additional pore size modification. Finally, the present invention claims gas separation beyond CO2/N2 and includes other, more difficult, separations.
U.S. Pat. No. 4,477,583 describes a method for depositing a coating of silica on a crystalline zeolite. The coated material is employed as a catalyst for the selective production of para-dialkyl substituted benzenes. This patent also refers to zeolites which specifically adsorb benzene, including the class of ZSM zeolites. The present invention differs from this prior art in that the coated zeolite is not used as a catalyst for benzene adsorption. This invention refers to the pore size reduction to <5 Å to facilitate a size selective adsorption of one gas over another. The '583 patent contains no reference to pore size consideration, and only generally refers to the use of the coating as a catalyst for benzene production. Additionally, the preferred zeolites are those having a framework density of not below 1.6 cubic centimeters. This would exclude the zeolite A, which has a framework density of 1.3 cubic centimeters. In the present invention, zeolite A is the most preferred zeolite to be used as the starting material for pore reduction, since the pore size is between 3 and 5 Å, depending of the cation type.
U.S. application 2013/0340615 A1 refers to adsorbent compositions using silicone-derived binding agents, which are shown to possess superior pore structures which enhance the rate of gas adsorption in the agglomerate. The properties of the final composition, including mean pore diameter, macropore size, and crush strength are addressed, but there is no mention of the change in micropore size of the zeolite as a result of the zeolite and silicon-derived binding agent mixture. In fact, this application does not acknowledge the advantages of the silicone as a tool for coating the individual zeolite crystals to be used as a means of modifying the pore size to facilitate the size selective separation of different gases.
U.S. application 2015/343417 discloses a method for modifying the surface of zeolites to form apertures smaller than 4.4 Å without a reduction of the pore volume. It specifically refers to the use of zeolite type A for drying moist refrigerants such as R11, 123, and R134a. It also refers to the use of tetra-ethyl-ortho-silicate as the modifying agent and the use of additional clay type binders to help bind the material to form agglomerates. As with the previously mentioned prior art, it does not address the use of silicone resins as the modifying agent and its' use as a binder, as well as the modifying agent. The present invention also has the additional feature of identifying the effect of the changing calcination temperature on the apparent pore size aperture and subsequently on the size selectivity. The present invention, as indicated in the following description and examples, has a more simplistic preparation, making it more amenable to a large scale commercial manufacturing processes.